Putative glucocorticoid receptors were measured in the brain and pituitary glands of young and aged rats in vivo and in vitro. Adrenalectomized rats were injected with a set dose of radiolabeled corticosterone plus unlabeled corticosterone; 2 h later [3H]corticosterone levels were measured in purified nuclear pellets from pituitary and 5 brain regions. Substantial decreases were seen in aged subjects in the maximal number of corticosterone binding sites in hippocampus and amygdala; all other regions showed no age-related changes. In contrast, there were no declines in the nuclear uptake of [3H]dexamethasone (DEX) in the aged rat brains. Since DEX interacts selectively with non-neuronal receptors in vivo, the deficit in glucocorticoid binding is selective for neurons. Subsequent studies assessing glucocorticoid receptor levels in cytosol preparations in vitro revealed significant declines in hippocampus and amygdala quantitatively comparable to the decline observed in nuclear binding in these loci. This suggests that the primary deficit leading to nuclear depletion may be the reduction of cytosolic receptor number, rather than other possible factors such as the reduction in receptor affinity or translocation of steroid-receptor complex. This decline may play a role in a number of limbic functions which are influenced by glucocorticoids and show deficits with age.